The present invention relates to a semiconductor laser and, more particularly, to a laser device construction which minimizes the stress applied to an active layer, thereby ensuring stable operation.
A GaAlAs semiconductor laser having the lasing wavelength in 0.8 .mu.m range has been developed, which enjoys an operating period above 10.sup.6 hours at room temperature. The operating life period is greatly lengthened by minimizing occurrence of a dark line or a dark spot, or by minimizing mirror erosion. The occurrence of the dark line or the dark spot is minimized by reducing the defect density in the grown crystal, which is achieved by the improvement of the crystal growth techniques or the reduction of oxygen in the growth system. The mirror erosion is minimized if the mirror surface is coated by a dielectric film made of, for example, Al.sub.2 O.sub.3, SiO.sub.2 or Si.sub.3 N.sub.4.
The above-mentioned semiconductor laser shows stable operation if the semiconductor laser operates as an infrared laser which has the lasing wavelength longer than 0.8 .mu.m. However, the semiconductor laser does not show stable operation nor the long operating period if the semiconductor laser operates as a visible laser which has the lasing wavelength shorter than 0.8 .mu.m.
To enhance the reliability of the GaAlAs semiconductor visible laser, improvements have been proposed in, for example, Japanese Patent Application Nos. 55-166124 and 56-44775, assigned to the same assignee as the present application, wherein the Te-doped or Se-doped cladding layer is formed through the use of the epitaxial method after the active layer has been formed, whereby the crystal construction of the active layer is greatly enhanced. The thus formed GaAlAs semiconductor laser exhibits the long operating period at the lasing wavelength around 0.77 .mu.m. However, the operating period becomes suddenly short as the lasing wavelength becomes shorter than 0.77 .mu.m.
Generally, in the GaAlAs double-heterostructure laser, the lattice constant of the GaAs substrate is similar to the lattice constant of the respective grown layers at the growth temperature, about 800.degree. C. However, the lattice constant of the respective layers and the lattice constant of the GaAs substrate differ from each other at room temperature, because the coefficient of the thermal expansion of Ga.sub.1-x Al.sub.x As varies as the Al mole fraction x varies. The above-mentioned difference of the lattice constant creates a large stress applied to the active layer at room temperature. The thus created stress may shorten the operating life period of the semiconductor laser. To minimize the stress caused by the mounted condition, a construction has been proposed in U.S. Patent Application Ser. No. 482,246, "SEMICONDUCTOR LASER" filed on Apr. 5, 1983, by Toshiro HAYAKAWA, Nobuyuki MIYAUCHI and Seiki YANO, and assigned to the same assignee as the present application (European Patent Application No. 83302006.8), wherein the active layer is separated from the mounted surface by at least a distance which corresponds to 35% of a thickness of the semiconductor laser element, and is separated from the opposing element surface by at least a distance which corresponds to 18% of the element thickness. However, sufficiently stable operation is not ensured by only controlling the location of the active layer.
Accordingly, an object of the present invention is to provide a semiconductor laser which stably emits the laser beam in the visible spectral range.
Another object of the present invention is to provide a semiconductor laser device structure, wherein the stress applied to the active layer is minimized.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
To achieve the above objects, pursuant to an embodiment of the present invention, the active layer is formed at a position separated from the mounted surface by a distance greater than 35% of the device thickness. Further, the active opposite to the mounted surface by a distance greater than 18% of the device thickness so as to minimize the stress applied to the active layer caused by the mounted condition. In order to minimize the strain caused by the difference of the coefficient of the thermal expansion of the respective layers, buffer layers having a thermal expansion similar to the cladding layers, which sandwich the active layer, are formed to contact the cladding layers.